Boron nitride
Boron nitride (BN) is a binary chemical compound, consisting of equal numbers of boron and nitrogen atoms. The empirical formula is therefore BN. Boron nitride is isoelectronic to the elemental forms of carbon and isomorphism occurs between the two species. That is boron nitride possess three polymorphic forms; one analogous to diamond, one analogous to graphite and ones analogous to the fullerenes. The diamond-like allotrope of boron nitride is one of the hardest materials known but is softer than materials such as diamond, ultrahard fullerite, and aggregated diamond nanorods. Polymorphs Boron nitride can be used to make crystals that are extremely hard, nearly as hard as diamonds, and the similarity of this compound to diamond extends to other applications. Like diamond, boron nitride acts as an electrical insulator but is an excellent conductor of heat. Like carbon, boron nitride exists in another polymorph that has structural and lubricating qualities similar to graphite. This form of boron nitride is composed of layers of fused hexagonal sheets (analogous to graphite). These sheets (unlike those in graphite) are in register. This means that layers are placed directly upon one another such that a viewer looking down onto the structure would view only the top layer. The polar B-N bonds interfere with electron transfer so that boron nitride in this form is not an electrical conductor (in contrast to graphite which is a semimetal that conducts electricity through a network of pi bonds in the plane of its hexagonal sheets). Boron nitride nanotubes can also be constructed analogously to carbon nanotubes. Image:Boron-nitride-(hexagonal)-side-3D-balls.png| α-BN, hexagonal Image:Boron-nitride-(hexagonal)-top-3D-balls.png| α-BN, hexagonal Image:Boron-nitride-(sphalerite)-3D-balls.png| β-BN, sphalerite structure Image:Boron-nitride-(wurtzite)-3D-balls.png| BN, wurtzite structure Cubic boron nitride The diamond-like allotrope of boron nitride, known as cubic boron nitride, c-BN, β-BN, or z-BN (after zinc blende crystalline structure), is widely used as an abrasive for industrial tools. Such usefulness is derived from the insolubility of boron nitride in iron, nickel and related alloys at high temperatures (unlike diamond). Like diamond, it has good thermal conductivity, caused by phonons; this is a difference against metals, where the mediators are electrons. In contact with oxygen at high temperatures, BN forms a passivation layer of boron oxide. Commercial products are known eg. under names Borazon (by Diamond Innovations), and Elbor or Cubonite (by Russian vendors). A crystal modification of boron nitride is w-BN, the superhard hexagonal phase of the wurzite structure. It occurs at high pressures. Polycrystalline c-BN (PcBN) is used for wear applications. It is superior to diamond in applications requiring high temperatures in oxidizing atmosphere, and contact with iron and its alloys; c-BN abrasives are therefore used for machining steel, while diamond abrasives are preferred for aluminium alloys, ceramics, and stone. Boron nitride binds well with metals, due to formation of interlayers of metal borides or nitrides. Materials with cubic boron nitride crystals are often used in the tool bits of cutting tools. Ceramic binders can be used as well. For grinding applications, softer binders, e.g. resin, porous ceramics, and soft metals, are used. Sintered cubic boron nitride can be used in electronics as an electrically insulating heatsink material. Cubic boron nitride is produced by treating hexagonal boron nitride at high pressure and temperature, much as synthetic diamond is produced from graphite. Direct conversion of hexagonal boron nitride to the cubic form occurs at pressures up to 18 GPa and temperatures between 1730-3230 °C; addition of small amount of boron oxide can lower the required pressure to 4-7 GPa and temperature to 1500 °C. Industrially, BN conversion using catalysts is used instead; the catalyst materials differ for different production methods, eg. lithium, potassium, or magnesium, their nitrides, their fluoronitrides, water with ammonium compounds, or hydrazine. Other industrial synthesis methods use crystal growth in temperature gradient, or explosive shock wave. The shock wave method is used to produce material called heterodiamond, a superhard compound of boron, carbon, and nitrogen. Low-pressure deposition of thin films of cubic boron nitride is possible. For selective etching of the deposited hexagonal phase during chemical vapor deposition, boron trifluoride is used (cf. use of atomic hydrogen for selective etching of graphite during deposition of diamond films). Ion beam deposition, Plasma Enhanced CVD, pulsed laser deposition, reactive sputtering, and other physical vapor deposition methods are used as well. Hexagonal boron nitride The graphite-like allotrope of boron nitride, known as hexagonal boron nitride, h-BN, α-BN, or g-BN (graphitic BN), and sometimes called white graphite, is useful as both a very low temperature and high-temperature lubricant (up to 900 °C in oxidizing atmosphere) and/or in situations where the electrical conductivity or chemical reactivity of graphite would be problematic. As the lubricity mechanism does not involve water molecules trapped between the layers, boron nitride lubricants can be used even in vacuum, e.g. for space applications. Due to higher electronegativity of the nitrogen atoms, the electrons that in graphite form a delocalized system, are concentrated around nitrogen atoms, sequestered outside the conductivity band, therefore not playing role in conductivity nor absorbing visible light. h-BN can be included in ceramics, alloys, resins, plastics, rubbers and other materials, giving them self-lubricating properties. Such materials are suitable for construction of e.g. bearings. Plastics filled with BN have decreased thermal expansion, increased thermal conductivity, increased electrical insulation properties, and cause reduced wear to adjacent parts. Hexagonal boron nitride is stable in temperatures up to 1000 °C in air, 1400 °C in vacuum, and 2800 °C in inert gas. It has one of the best thermal conductivities of all electric insulators. It is fairly chemically inert and is not wetted by many melted materials (e.g. aluminium, copper, zinc, iron and steels, germanium, silicon, boron, cryolite, glass and halide salts). h-BN parts can be made by hot-pressing with subsequent machining; due to the mechanical hardness similar to graphite, the machining cost is low. The parts are made from boron nitride powders, using boron oxide as a sintering agent. Addition of boron nitride to silicon nitride ceramics improves the thermal shock resistance of the resulting material. For the same purpose, BN is added also to silicon nitride-alumina and titanium nitride-alumina ceramics. Other materials being reinforced with BN are e.g. alumina and zirconia, borosilicate glasses, glass ceramics, enamels, and composite ceramics with titanium boride-boron nitride and titanium boride-aluminium nitride-boron nitride and silicon carbide-boron nitride composition. Due to its excellent dielectric and insulating properties, BN is used in electronics e.g. as a substrate for semiconductors, microwave-transparent windows, structural material for seals, electrodes and catalyst carriers in fuel cells and batteries. Fine-grained h-BN is used in some cosmetics, paints, dental cements, pencil leads, etc. Hexagonal boron nitride is produced by the nitridation or ammonolysis of boron trioxide. Thin films of boron nitride can be obtained by chemical vapor deposition from boron trichloride and nitrogen precursors. Industrial production is based on two reactions: melted boric acid with ammonia, and boric acid or alkaline borates with urea, guanidine, melamin, or other suitable organic nitrogen compounds in nitrogen atmosphere. Combustion of boron powder in nitrogen plasma at 5500 °C is used for production of ultrafine boron nitride for lubricants and toners. Boron nitride fibers Hexagonal BN can be prepared in the form of fibers, structurally similar to carbon fibers, sometimes called white carbon fiber. Two of the main methods of their synthesis are thermal decomposition of extruded borazine fibers with addition of boron oxide in nitrogen at 1800 °C, and thermal decomposition of cellulose fibers impregnated with boric acid or ammonium tetraborate in the mixture of ammonia and nitrogen above 1000 °C. Boron nitride fibers are used as reinforcement in composite materials, with the matrix materials ranging from organic resins to ceramics to metals (see Metal matrix composites). Boron nitride nanotubes Boron nitride nanotubes (BNNT) have been prepared, and like BN fibers, show promise for aerospace applications where integration of boron and in particular the light isotope of boron (10B) into structural materials improves their radiation-shielding properties, due to 10B's neutron absorption properties. Such 10BN materials are of particular theoretical value as composite structural material in future manned interplanetary spacecraft, where absorption-shielding from cosmic ray spallation neutrons is expected to be a particular asset in light construction materials.http://wwwrsphysse.anu.edu.au/nanotube/bnnts.php Boron nitride nanomesh Boron nitride nanomesh is a new inorganic nanostructured two-dimensional material. It consists of a single layer of hexagonal boron nitride on rhodium or ruthenium, forming a highly regular mesh. The distance between two pore centers is 3.2 nanometers (=0.0000032 mm!) and the pores are 0.05 nanometer deep. The boron nitride nanomesh is stable under vacuum, air and some liquids, but also up to temperatures of 796 oC. In addition, it shows the extraordinary ability to trap molecules and metallic clusters. These characteristics promise interesting applications of the nanomesh in nanotechnology. Amorphous boron nitride Layers of amorphous boron nitride (a-BN) are used in some semiconductor devices, eg. MISFETs. They can be prepared by chemical decomposition of trichloroborazine with caesium, or by thermal chemical vapor deposition methods. Thermal CVD can be also used for deposition of h-BN layers, or at high temperatures, c-BN. Rhombohedral boron nitride Rhombohedral boron nitride is similar to hexagonal boron nitride. It is formed transitionally during conversion of cubic BN to hexagonal form. Other allotropes The fullerene-like allotropes of boron nitride can be synthesized and resemble those of carbon. The recently discovered boron nitride nanotubes are an important development due to their homogeneous electronic behavior. That is, tubes of different chiralities are all semiconductor materials with the same (approximate) band gap. See also *Beta carbon nitride *Borazon *Boron phosphide *Boron suboxide *Aluminium nitride *Wide bandgap semiconductors External links *National Pollutant Inventory: Boron and Compounds *Fiz Chemie Berlin thermophysical database *Materials Safety Data Sheet at University of Oxford cs:Nitrid boritý de:Bornitrid es:Nitruro de boro eo:Bornitrido fr:Nitrure de bore it:Nitruro di boro nl:Boornitride ja:窒化ホウ素 pl:Azotek boru pt:Nitreto de boro zh:聚氮化硼 Category:Nitrides Category:Boron compounds Category:Ceramic materials Category:Superhard materials Category:Lubricants Category:Semiconductor materials Category:III-V compounds